International Academy of Wood Science

ACADEMY LECTURE

WOOD AS NATURAL SMART MATERIAL

Boris UGOLEVMoscow State Forest University, Russian Federation

Saint-Petersburg Moscow, Russia2009

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It will not be too pessimistic to state that we shall not learn to know the wood ultrastructuretruths before 2050 or perhaps 3000. Anyhow it is most fascinating science to deal with .

A. Bjorkman Wood Science1920-2003-?

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Nature that created wood as ultramicroscopic miracle, prompted to mankind to produce diverse modern artificial materials ranging from reinforced concrete to nanocomposites. Further studies of wood nanostructure discover new possibilities of biomimetic approach to creation of effective materials.

Material scientists predict a prominent role of artificial smart materials in future. This term designates material which usefully reacts to the changes in environmental parameters.

One of the main features of smart materials is shape-memory effect. It means that these materials after forced change of the form are able to restore their initial form when original physical condition is recovered.

Metallic shape memory alloys were synthesized the first. Later were created ceramics and polymers with the same property.

We find a very broad spectrum of smart materials applications from deployable space structures to self-repairing auto bodies, switches, sensors, kitchen utensils, tools up to minimally invasive surgery and implants in biomedicine.

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fiber-reinforced elastic memory composites (Abrahamson et al. 2002)

problems of behaviour of the shape memory polymers (Lendlein and Kelch 2002)

( )

Examples of Topical Problems for Artificial Smart Materials

internal stress induced by constrained expansion of memory polymer nanocomposites (Gall et al. 2004)

model development for shape memory polymers(Siskind et al. 2008)

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Hygroscopicity of wood gives possibility to use it as a sensor of surrounding air humidity.

Wood constructions of buildings regulate, in some degree, the air humidity in living premises, by drying or humidifying of the air at changing weather.

Swelling of wood tightens joints in articles and structures.

Pit props of pine and spruce wood emit crackling which warns of coming mine destruction.

Acoustic emission is used for monitoring of lumber stress state at drying.

Sonorous ability of wood reveals itself in musical instruments.

Piezoelectric properties of wood permit to create nondestructive methods of strength testing.

Et cetera.

( , )

Examples of Wood Useful Properties under External Influence

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The notion memory of wood was introduced by us at the beginning of the 1980s. Previously some researchers (. Takemura (1973) et al.) had used the analogous term for indication of purely temporal phenomena. In our case this is a metaphor which reflects the ability of wood to react to the restoration of initial physical state determined by its moisturecontent and temperature.

Wood memory effect is based on quazi-residual frozenstrains

They were experimentally discovered by us at constrained shrinkage of wood in the early 1960s.

(FS)

( )

Wood Possesses Memory Effect The Dominant Feature of Smart Materials

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= W + W+ + 0

21 ),(),( TWEHd

TWEH

&

Integral Law of Wood Straining at Drying and Wetting

Here: strain coefficient of shrinkage

W=w moisture content decrease from limit saturation of cell wall (FSP) coefficient of swelling stressE stiffness modulus temperature decrease from 100CW(),T() and are functions of time H1 and H2 are Havisade`s functions accordingly for drying and wetting

)(&

( )

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Here e elastic strain; v viscous strain; ev elastic- viscous strain; c creep; stress

ec

v

ec

ev

====const ====0

Strain () Time () Relation of Wood at Constant Moisture Content

( () () )

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Here: stress; strain; w drop of moisture content.1 wet wood ( w > 30%)2 dry wood

1

2

4 1 1 9 7

0 1 8 6 5

10

1 2

W

0 3 1' 2'

Changes of Wood Hygromechanical Strains( )

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Changes of Strains , Stresses and Moisture Content w in Time of Experiment

1 wet wood2 dry wood

,h

( , w )

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Scheme of Forming of Frozen Elastic-ViscousStrain at Wood Drying

f = ev1 ev2

Frozen strains are the result of temporary reconstruction of wood nanostructure under the control load influence at increasing wood stiffness processes of drying or cooling

( - )

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Contribution of Frozen Strains to Stress-Strain State of Wood at Drying

The FS as a part of set-strain are the main reason for the forming of dried lumber casehardening

s = f + r

( - )

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Frozen Strains Were Taken into Account at the Drying Stress Calculation

( )15,0 += jj EEEw

stress coefficient of shrinkageE stiffness modulus

average stiffness modulus moisture contents drop from limit of cell wall saturation (FSP)

j step numberi slice numbern quantity of slices

Here:

+= = =

n

i

n

i

ji

ji

ji

ji

ji

ji

ji EwEwE

1 1

1 /

Step-by-step method for multy-sliced model of drying lumber

( )

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Lumber drying schedules parameters

Product quality parameters

temperature

humidity

time

moisture content

moisture content gradient

stress

Lumber Drying Schedules and Product Quality( )

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Detection of Lumber Drying Stresses

( )

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Method of Lumber Drying Stresses Monitoring by Differential Shrinkage (DS)

( )

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Scheme of Wood Deformative Conversions at Heating

0

5

1

2

6

3

8

4

7

=const

0=const

f t1 r

R

Here: = 100 - t drop of temperatureR = t(0 f ) at 0 = const (case a)r = t( f ) at = const (case b)t modulus of elasticity

( )

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Wood Stress Memory Effect at Heating

loaded wood and constanttotal strain

unloaded wood and constantfrozen strain

0

0,2

0,4

0,6

20 40 600

0,4

0,8

1,2

20 40 60

1

3 2

6

4

32

7

1

8

, P , P

Rr

t, Ct, Ca b

Pinus sibirica Du Tour, compression, tangential direction across the grain,

0=0,009

Quercus robur L., tension ,radial direction across the grain,

=0,007

( )

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Change of the Form of Ash Veneer and Strain Memory Effect

a) original state (heated wood)

b) under load c) after cooling

d) after unloading( frozen strains)

e) after heating

( )

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The Scheme of the Wood Specimen Loading

b= 10

1 40

t

r I

h=10

l =100

II

( )

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Strain Memory Effect at Heating of Previously Loaded (Tension Compression) and then Unloaded Wood

-0,002

-0,001

0

0,001

0,002

0,003

0,004

0,005

0,006

0,007

20 40 60 t , C 1

6

2

3

2

7 5

4 4

)

b) After loading cooling unloading

After heating

( ( ) )

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Contribution of Hygrofrozen Strains (HFS) and ThermofrozenStrains (TFS) in Complex Rrozen Strains (CFS)

Complex frozen strain (CFS)

Wood drying and cooling under loadconsecutive simultaneous

HFS

TFSTFS

CFS+SESCFS

HFS

( (HFS) (TFS) (CFS))

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Using Hygro-Thermofrozen Strains to RemoveVeneer Waviness

02468

1012

0 30 60 90 120 150 180 210 240

Width of specimen, mm

Hei

ght o

f w

aves

, m

m 1

2

1 original state, 2 after cooling and drying of loaded specimen.

( - )

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Wood Stress-Strain Behavior of the Surface Zone of a Board at Drying and Conditioning Moisture-Heat Treatment

0

1

3

2

4

-

+

+

-

(-

-)

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IR- Absorbtion Spectrum of Birch Wood

- - - - after free drying after constraind shrinrage

(- )

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Experimental Device

( )

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Scheme of Wood Deforming at Constant Load, Drying and Unloading

f

f

v + e c s *

f v c f e s

-

*

7 2

2 2

(2) 1 0

5 6 7 3 4 0 (4)

evc

Ee2 Eev1 Eev2

Ee2> Eev2 >Eev1

Here 0-1 tension of wet wood1-2 drying of loaded wood2-3-4unloading and time exposure of dry woodsections0-4 reduced shrinkage *

4-5 frozen elastic-viscous strain f 5-6 creep s4-6 quasi-residual set-strain s

6-7 frozen shrinkage f0-7 free shrinkage

( , )

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Effect of Tension Load on Reducing Shrinkage Degree

1 Fraxinus excelsior L., t = 80 C (our date)2 Fagus orientalis, t = 20C (by N.Chulitsky)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

K=*/

0 0,2 0,4 0,6 0,8 1 1,2 1,4 , MPa

1

2

( )

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Detection of Frozen Strain and Frozen Shrinkageat Drying Fastened Specimen

0

0,01

0,02

0,03

0,04

0,05

2 4 6 8 10 , h

0

30

60

90

0 2 4 6 8 10

0

1

2

0 2 4 6 8 10

- ,,,, ,,,, -

, P

W, %

W fsp

, h

, h

f

2 **

f

c

2

3 4

5

1

6 * 6

2 *

6 **

7

7

6 (5 ) 4

1

2

3

1 2 3

4

5 6 7

* Fraxinus excelsior L., tangential direction across the grain, t = 80 C

( )

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Free Shrinkage and Stress-Strain State at Wood Drying

1

2

3

4

5

-0,05 -0,04 -0,03 -0,02 -0,01 0 0,01 0,02 0,03 0,04 0,05

,

- 1 2 3

4

5

6 6** 6*

f

'

4

2*

ev=* c f

Fraxinus excelsior L., tangential direction across the grain, t=80 C

( - )

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=

=n

iii twEwK

1

),()(

Drying Stress Calculation Equation with Reducing Shrinkage Coefficient

Here coefficient of shrinkagew moisture content drop from limit saturation of cell wall (FSP) stressE stiffness modulust temperature decrease from 100 Cw moisture content

( )

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Decreasing of Wood Stiffness at Hygrofatigue

n number of sorptiondesorption cycles: MC from 12 to 20 %Picea abies; tangential direction across the grain

( )

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Wood Deformative Conversions at Change of Moisture and/or Content or Temperature

( / )

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Wood Strains at Drying or Wetting( )

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Classification gives an opportunity to consider it as a peculiar Mendeleev's table and detect blank spots, lay out

research programs. Further research of deformativeconversions will permit to improve wood technology and

create new smart wood composites.

Thank you for your attention